As a solder resist patterning method for a print wiring board, alkali developing methods are widely used due to the excellent chemical or solvent resistance of the cured film. Thus, for an alkali-developing-type resist ink that is used for this alkali developing method, an acid-pendant-type epoxy vinyl ester compound obtained by reacting an unsaturated monocarboxylic acid with an epoxy resin and then adding a polybasic anhydride thereto is generally used as a main component.
Recently, as an exposure system employed for a solder resist patterning method by the alkali developing method, a direct exposure method by a laser beam (laser direct imaging method) has been proposed and attracted considerable attention. The direct exposure method using a laser beam is characterized by the formation of a more precise pattern compared to a conventional UV exposure method using a mask pattern. Therefore, it is an exposure method useful for making smaller or integrated base boards.
However, exposure energy obtained by a laser beam can be as small as about 1/10 to 1/100 of that of an UV ray. Sufficient curability cannot be attained with one of the alkali-developing-type resist ink used in the past. Accordingly, there is a need for a resist ink having ultrahigh sensitivity that can be used for such a direct exposure method using a laser beam.
Moreover, even in a current system in which curing is performed by a conventional type of mask pattern using a UV exposure method, the exposure energy during the solder resist patterning tends to decrease as the speed of production lines is increased. As a result, there is increasing need for a resist ink that has ultrahigh sensitivity and that exhibits excellent curability even if low exposure energy is used.
As such, as a solution to such higher sensitivity requirements, for example, a technique for achieving a highly sensitive photosensitive resin, in which an epoxy group-containing radically-polymerizable unsaturated monomer (one representative example of which is glycidyl methacrylate) is reacted with an acid-pendant-type epoxy vinyl ester compound obtained by reacting an epoxy resin with an unsaturated monocarboxylic acid and a dibasic anhydride to introduce a multiplicity of unsaturated double bonds into the polymer structure, thereby imparting high sensitivity thereto (see, for example, Patent Document 1).
However, when a vinyl ester resin is prepared by reacting an epoxy group-containing radically-polymerizable unsaturated monomer with an acid-pendant-type epoxy vinyl ester compound, the epoxy group-containing radically-polymerizable unsaturated monomer is reacted with a carboxylic group in the acid-pendant-type epoxy vinyl ester compound whereby acid groups therein are consumed. As a result, this could not avoid a reduction in alkali developability. Further, when the epoxy group-containing radically-polymerizable unsaturated monomer is blended with the vinyl ester resin to provide more ultrahigh sensitivity, not only is the development rate remarkably lowered, but a “heat control range” applicable in predrying also becomes extremely narrow, thus being unfavorable. The term “heat control range” applicable in the predrying refers to a thermally acceptable limit in the heat drying where the coating film is heat-dried, and then, an unexposed portion of the film is developed, or a control range of the heat-drying conditions that is allowable to maintain developability. Thus, if the heat control range is narrow, the resin composition of the coating film curing under time or temperature conditions outside the acceptable range during the drying process where a solvent is removed after an alkali-developing-type resist ink is applied to a print wiring board and the solvent is removed. Consequently, even if the film is exposed and developed after the drying process, it is difficult to remove the unexposed portion due to the presence of the developing solution. Therefore, a problem has been caused in that the solvent remains thereon even after the development, thereby causing tackiness, since sufficient drying cannot be performed.
As described above, such a type of vinyl ester resin obtained by reacting a conventional epoxy group-containing radically-polymerizable unsaturated monomer with an acid-pendant-type epoxy vinyl ester compound causes deterioration in the alkali developability and the narrower heat control range in predrying when the level of modification of the epoxy group-containing radically-polymerizable unsaturated monomer is increased to achieve the above-described ultrahigh sensitivity that has been sought in recent years because the acid value required for the alkali developability and the number of unsaturated double bonds that contribute to the sensitivity become inversely proportional to each other. On the other hand, if the level of modification is suppressed to increase the alkali developability and the heat control range in predrying, the ultrahigh sensitivity, which has been sought in recent years could not be attained.
Accordingly, the field of alkali-developing-type resist ink is still in a situation where the ink is not applicable to the direct exposure method using a laser beam, and where it is impossible for the ink to have ultrahigh sensitivity that can exhibit excellent curability even if low exposure energy is applied thereon; excellent developability; and a broad heat control range, simultaneously.    Patent Document 1: Japanese Unexamined Patent Publication No. 10-282665 (pages 3 to 5)